feat: track CSS custom properties on CSSSourceCode

[Copilot]

Implements the CSS variable tracking RFC, centralizing custom property tracking on CSSSourceCode so rules don't duplicate this logic. Also fixes variable hoisting — var() references before their declaration now resolve correctly.

CSSSourceCode — new API

Three new public methods backed by private #customProperties Map and #declarationVariables WeakMap, populated during traverse():

• getDeclarationVariables(declaration) — returns var() function nodes used in a declaration's value
• getClosestVariableValue(func) — returns the closest value for a var() node. Resolution priority per RFC: same-block declaration → fallback → other-block declarations (hoisted) → @property initial-value → undefined
• getVariableValues(func) — returns all known values: @property initial-value, then declarations in source order, then fallback

// Rule usage example
const closestValue = sourceCode.getClosestVariableValue(varFuncNode);
const allValues = sourceCode.getVariableValues(varFuncNode);
const varsInDecl = sourceCode.getDeclarationVariables(declNode);

no-invalid-properties — refactored to use new API

• Uses getClosestVariableValue() for primary resolution instead of maintaining its own variable map
• Populates internal vars map incrementally via rule visitor methods; hoisting is handled by getClosestVariableValue() which has access to all declarations collected during CSSSourceCode.traverse()
• Falls back to direct chain resolution when closest value contains unresolvable nested var() refs
• 3 test cases move from invalid → valid: when a var() has a valid fallback, the fallback is now preferred over an invalid value from another selector block

Tests

• 19 new tests covering all three CSSSourceCode methods (same-block, fallback priority, hoisting, @property, multiple declarations, cross-declaration isolation)
• 2 hoisting tests for no-invalid-properties

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[eslint-github-bot]

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[nzakas]

@copilot the rule itself is doing a depth-first traversal of the AST. Doing it again is inefficient. Remove the extra traversal and use the rule methods to do any traversing that is necessary.

[Copilot]

Removed the walkAST function and StyleSheet() visitor. The vars map is now populated incrementally in "Rule > Block Declaration:exit" as the rule traverses. Hoisting is handled by sourceCode.getClosestVariableValue() which has access to all declarations collected during CSSSourceCode.traverse(). Commit: 63d930c